A phosphor-coated blue LED array, for example a GaN-based LED array, together with a red LED array, for example a AlInGaP LED array, are widely used in efficient LED lamps to generate warm white light in a low CCT range, for example from 2500K to 3000K, for the benefit of high luminous efficacy as well as good CCT and CTI.
The blue LED array and the red LED array have different lumen degradations as a function of junction temperature of the blue LED array and the red LED array, i.e. the red LED array has a much faster lumen degradation than the blue LED array as a function of junction temperature. Therefore, the junction temperature of the blue LED array and the red LED array after the LED lamp is activated will be controlled to a specific temperature, for example 80° C., which is referred to as the thermal stable temperature, to ensure the LED lamp generates the desirable warm white light.
The light generated by the LED lamp after being activated is more reddish initially and then gradually shifts to the desirable warm white color as the junction temperature of the blue LED array and the red LED array increases. Generally, after being activated, the LED lamp will take 20 minutes or even more to achieve the thermal stable temperature, and the user may notice the color shift, for example from reddish to the desired warm white, and feel uncomfortable during this long thermal stable time.